With the development in digital technologies in recent years, electronic devices such as mobile information equipment and information home appliances have higher functionality. Thus, demands for increasing the capacity of nonvolatile storage elements included in these devices, reducing the write power, accelerating writing and reading operations, and increasing the life span of these devices have been increasing.
To meet such demands, miniaturization of flash memories including existing floating gates is said to have limitations. Thus, attention is recently focused on a new variable resistance nonvolatile storage element including a variable resistance layer as a material of a storage unit.
The variable resistance nonvolatile storage element has a very simple structure including a variable resistance layer that is disposed between a lower electrode and an upper electrode. A resistance state of the nonvolatile storage element changes between a low resistance state and a high resistance state only with application, between the lower electrode and the upper electrode, of a predetermined electric pulse having a voltage higher than or equal to a threshold. Then, information is recorded in association with these different resistance states and its values. Since the variable resistance nonvolatile storage element (also referred to as “variable resistance element”) has such a simple structure and simply performs operations, it is expected that the nonvolatile storage element can further be miniaturized and the cost can be reduced. Since the resistance state of the variable resistance nonvolatile storage element sometimes changes between the low resistance state and the high resistance state by orders of magnitude not longer than 100 nanoseconds (ns), the attention is further focused on the variable resistance nonvolatile storage elements in view of its higher operating speed, and various proposals of these have been made.
In particular in recent years, there are many proposals for variable resistance nonvolatile storage elements comprising metal oxides in the variable resistance layers. Such variable resistance nonvolatile storage elements comprising metal oxides can be largely divided into two types, depending on a material to be used in each of the variable resistance layers.
One type is the variable resistance nonvolatile storage elements comprising perovskite materials (Pr(1-x)CaxMnO3 [PCMO], LaSrMnO3 [LSMO], and GdBaCoxOy [GBCO], for example) in the variable resistance layers, as disclosed in PTL 1 and others.
The other is the variable resistance nonvolatile storage elements that are compounds comprising only transition metals and oxygen, using binary transition metal oxides. Compared to the perovskite materials, the binary transition metal oxides have very simple composition structures. Thus, controlling the compositions when manufactured and in forming the films are relatively easy. In addition, with the advantage of relatively favorable compatibility with semiconductor manufacturing processes, the variable resistance nonvolatile storage elements have intensely been studied in recent years. For example, PTL 2 discloses variable resistance elements comprising, as variable resistance materials, (i) transition metal oxides of stoichiometric composition, such as nickel (Ni), niobium (Nb), titanium (Ti), zirconium (Zr), hafnium (Hf), cobalt (Co), iron (Fe), copper (Cu), and chrome (Cr), and (ii) oxides whose composition is deficient in oxygen compared to its stoichiometric composition (hereinafter referred to as oxygen-deficient oxides). Furthermore, PTL 3 discloses a nonvolatile storage element comprising an oxygen-deficient tantalum (Ta) oxide as a variable resistance material. When a Ta oxide layer is denoted as TaOx, PTL 3 reports a resistance change phenomenon in a range satisfying 0.8≦x≦1.9 (from 44.4 to 65.5% in terms of oxygen concentration).
Furthermore, PTL 3 also reports that a variable resistance nonvolatile storage element has two different operation modes, namely, unipolar (monopolar) switching and bipolar switching.
The unipolar switching is an operation mode in which a resistance value changes with application of electric pulses having the same polarity and different amplitudes between a lower electrode and an upper electrode of a variable resistance nonvolatile storage element, which is disclosed by PTL 2 and others. Furthermore, the unipolar switching requires changing not only the magnitude of the voltage but also the length (pulse width) of the electric pulse simultaneously, as disclosed by PTL 4 in detail. For example, the unipolar switching requires using two types of electric pulses, that is, a nanosecond pulse and a microsecond pulse.
In contrast, the bipolar switching is an operation mode in which a resistance value changes with application of electric pulses of positive and negative polarities between a lower electrode and an upper electrode of a variable resistance nonvolatile storage element, which is disclosed by PTLs 1 and 2. As disclosed by PTL 4, electric pulses of a nonvolatile storage element that performs the bipolar switching are generally set to the same length of order of nanoseconds.